Flat-panel displays, including a liquid crystal display and a plasma display, have been applied to a wide variety of apparatuses from a mobile device such as a portable telephone terminal to a large apparatus such as a public display installed on the street. In most of such displays, the focus of development is placed on points such as wide viewing angle, high brightness, and high image quality, and a display that is clearly and easily visible from any angle has been demanded.
On the other hand, contents displayed on a display include contents such as secret information and/or private data not desired to be seen by other people. Hence, in the current circumstances in which ubiquity is advancing with the development of information devices, it is also an important challenge to prevent other people to see displayed contents even in a public situation in which unspecified people are present.
In addition, even at a place such as an office where only specified people are present, there are occasions when a person handles secret information not desired to be seen by a person passing through behind his or her seat.
Devices such as portable telephone terminals include a device equipped with a display that allows displayed contents to be viewed only from a specific direction by providing an optical shield plate (louver). However, displayed contents can be still stealthily seen from directly behind a user, and therefore, such a device is not adequate for providing security protection.
Techniques related to the solution of these problems include an “image display apparatus” disclosed in Patent Document 1. In this image display apparatus, a user is made to wear an eyeglass having an image selection function, thereby allowing a specific image (hereinafter, referred to as a “secret image”) to be viewed only by a person (user) who wears the eyeglass, and presenting another image (hereinafter, referred to as a “public image”) to other people.
Specifically, the image display apparatus shown in FIG. 1 accumulates an input image signal 11 for a single frame in an image information accumulation memory 12 based on a frame signal 13. Thereafter, image information is read from the memory 12 at a speed twice as fast as that of a frame period (i.e., image information is read twice during a single frame period), the initially read signal is compressed to one-half and inputted as a first image signal 14 to a synthesis circuit 15, and the subsequently read image signal is subjected to chroma and brightness conversion and then inputted as a second image signal 17 to the synthesis circuit 15. Accordingly, the first image signal 14 and the second image signal 17 are alternately displayed on an image display device 18.
On the other hand, the frame signal 13 is also inputted to an eyeglass shutter timing generation circuit 19. The eyeglass shutter timing generation circuit 19 drives a shutter of an eyeglass 21, and controls the eyeglass shutter so as to prevent an image formed based on the second image signal 17 from being seen from a user.
With such configuration and operation, a person who is not wearing the eyeglass 21 will see a gray image or a third image (public image) which is a synthesized image resulting from the first image signal 14 and the second image signal 17 and irrelevant to the first image signal 14, while a person who is wearing the eyeglass 21 will see a desired image (secret image) formed based on the first image signal 14.
Further, the other techniques related to the solution of the above-mentioned problems include a “Secure method for providing privately viewable data in a publicly viewable display” disclosed in Patent Document 2. The method disclosed in Patent Document 2 allows only an authorized user to decipher a private image (secret image) on a display, and at the same time presents, as a public image, an image such as a simple random pattern or unreadable pattern or a screen saver image to an unauthorized user.
In order to promote this object, in the invention disclosed in Patent Document 2, an image processing technique including a data hiding pattern and an alternating pattern is synchronized with a display into which an image formed by the image processing technique is incorporated (for example, the image processing technique is combined with a wearable device such as active glasses). Finally, with the use of a human visual system's “known ability to merge a dissimilar image into a single image”, the ability to provide data viewable privately on a display viewable by general public is completed.
Thus, in the inventions disclosed in Patent Document 1 and Patent Document 2 described above, images are switched at a high speed; therefore, so as not to allow a flicker to be sensed, the frame frequency needs to be equal to or greater than a value obtained by the following formula: (Number of Images Switched)×50 Hz.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 63-312788
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-255844